A Prof Hamish Meffin
hmeffin@unimelb.edu.au
Department of Biomedical Engineering
Lecture 6.3: Auditory System
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Semicircularducts
Vestibular nerve
Cochlear nerve
Vestibulocochlearnerve (VIII)
Internal
auditory meatus
Cochlea
Figure 14.34
Pinna
External auditory
meatus
Tympanicmembrane
Malleus
Incus
Stapes
Outer ear
Middle ear
Inner ear
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Ear Anatomy & Physiology
Outer Ear(pinna and ear canal): Has air medium
Serves to direct and transmit sound.
TympanyMembrane
Transduces air vibration into mechanical vibration and amplifies the intensity ofthe vibration.
Middle Ear(malleus, Incus, Stapes)
If there is no impedance balancing of the air medium from the outer ear and theliquid medium of the inner ear, only ~0.1% of the incident power of the soundwaves would be transmitted (i.e., an attenuation of 30 dB).
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Ear Physiology
Transduction of sound into an auditory perception
Sound is a propagating pressure wave.
Perception of sound involves the electrical activity of neurons in the auditorycortex of the brain.
The transduction process is the means by which the pressure waves in air (amechanical stimulus) is converted into neural activity (action potentials).
This process involves a number of stages, some of which involve conductionand impedance matching.
The path of sound
ear canalvibrate tympanic membrane
vibrateossicles(3 bones: Malleus, Incus, Stapes)
vibrate oval window of cochlea
create waves in cochlea fluid
create standing waves in basilar membrane
movement of hair cells generates electrical activity through mechanicallygated ionic channels
hair cells stimulate the auditory nerve
series of action potentials up to the auditory cortex.
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Cochlear anatomy
Kiang et al., 1974
AN
CNS
cochlea
Organ ofCorti: The transducer
Vibration of the basilar membrane produces shear forces thatbend the stereocilia (hairs protruding from the hair cells)against the tectorial membrane
Movement of the stereocilia either cause the hair cell todepolarise or hyperpolarise, depending upon the direction ofmovement
Changes in the membrane potential of the hair cell generate an APin the nerve fibre attached to the hair cell.
Electron Micrograph: Apparent are inner hair cell and outer haircell and rows ofstereocillia
Scala tympani
Rosenthal’s
canal
Spoendlin, 1984
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Inner Hair Cells
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Resting Electrical Potentials of the Cochlear
Two classes of potential: Resting potentials and stimulus-related potentials
Maintenance of large DC potentials between endolymphatic andperilymphaticspaces plays an important role in active cochlear tuning
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Inner Hair Cell Receptor Potential
Below 1000 Hz–follows oscillations
Above 3500 Hz–follow envelope only
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Cochlea mechanics: Active processes
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Active Cochlea Mechanics
Scala tympani
Rosenthal’s
canal
Spoendlin, 1984
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Frequency Tuning Curve
log frequency
100
80
60
40
20
Characteristic
Frequency
Threshold
Bandwidth
log amplitude (dB SPL)
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The discharges of cochlear nerve fibres to low-frequency sounds arenot random;
they occur at particular times (phase locking).
Evans (1975)
Temporal Frequency Coding
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Temporal Coding
Information is contained in the relative timing of individual action potentials.E.g.
The temporal code contains detailed information about the phase of a periodicsignal, i.e. much more fine-grained information about the stimulus down to aresolution of ~10 us at a population level.
Note: This information is in addition to rate information, not in contradiction ofit.
Temporal information is observed in the auditory pathway, but diminishes inresolution from auditory nerve to auditory cortex (~10 us to ~10 ms)
Does temporal information play a functional role or is it an epiphenomenon?
What evidence is there that the temporal code is used by the brain?
Outer Ear: amplification and direction filtering.
Middle Ear: impedance matching.
Cochlear & Basilar Membrane: spectral analyser.
Organ ofCorti: Acoustic to neuro-electric transducer.
Inner Hair Cells: Synapse type I AN fibres projectingcentrally.
Outer Hair Cells: Amplification and improved frequencytuning.
Auditory Nerve: Carries signal to the auditory braincentres. Information is convey by both rate and timing ofaction potentials.
Dorsal
Ventral
Rostral
Caudal
Auditory Pathway
Figure from MIT OpenCourseWare
lateral
rostral
dorsal
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Octopus
Spherical bushy
Globular bushy
Fusiform
Stellate
Stellate
Ventral
Anterior
Posterior
Dorsal
DCN
AVCN
PVCN
AN
Cochlear Nucleus
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Parallel pathways emerge from the CN
VCN extracts and enhances timing and spectrum, as well as information on thewaveform envelope (e.g., amplitude modulation)
Bushy cellsTBSOCDNLLIClocalisation“where”
T-stellate cellTBVNLLICspectrum, AM“what”
Octopus cellVNLLICtemporal pattern
DCNIClocalisation & spectrum
monaural pinna (spectral) cues
These parallel streams converge in the central nucleus of the inferior colliculus (IC)in the midbrain
Abbreviations:
TB–Trapezoid body
SOC–superior olivary complex
DNLL–dorsal nucleus of the lateral lemniscus
VNLL–ventral nucleus of the lateral lemniscus
IC–interior colliculus
Dorsal
Ventral
Rostral
Caudal
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Auditory Cortex
from http://www.physiology.wisc.edu/neuro524/audition02-524.htm
Left hemisphere is specialised in the analysis, perception and production oflanguage.
Right hemisphere implicated in the perception of duration, intensity,intonation, melody.
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Laminar Structure
From: Fundamental Neuroscience (2002), Elsevier, Fig 8, CH 22
Supragranular
layers (1-3)
Granular layer (4)
Infragranular
layers (5-6)
stellate cell
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Structure of Cerebral Cortex
Smith & Populin, 1977
Both anatomical and electrophysiological evidence supports acolumnar organization of auditory cortex.
Neurons in vertical electrode penetrations have similar responseproperties. Eg. CF, binaural response, bandwidth.
Synaptic connections between neurons with similar responseproperties
Binaural interaction map
Spectral integration map
Read et al., 2002
Smith & Populin, 1977
Spatial clustered or topographic arrangement of neural responseacross the cortical surface.
Cortical maps of:
Frequency (tonotopic)
Binaural response type
Bandwidth.
Binaural and bandwidth maps are roughly orthogonal to tonotopicmap.
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Summary of the Auditory Pathway
Tonotopic organisation